A photoelectric smoke detector system measures the ambient smoke conditions of a confined space and activates an alarm in response to the presence of unacceptably high amounts of smoke. This is accomplished by installing in a housing covered by a smoke intake canopy a light-emitting device ("emitter") and a light sensor ("sensor") positioned in proximity to measure the amount of light transmitted between them.
A first type of smoke detector system positions the emitter and sensor so that their lines of sight are collinear. The presence of increasing amounts of smoke increases the attenuation of light passing between the emitter and the sensor. Whenever the amount of light striking the sensor drops below a minimum threshold, the system activates an alarm.
A second type of smoke detector system positions the emitter and sensor so that their lines of sight are offset at a sufficiently large angle that very little light propagating from the emitter directly strikes the sensor. The presence of increasing amounts of smoke increases the amount of light scattered toward and striking the sensor. Whenever the amount of light ago striking the sensor increases above a maximum threshold, the system activates an alarm.
Because they cooperate to measure the presence of light and determine whether it exceeds a threshold amount, the emitter and sensor need initial calibration and periodic testing to ensure their optical response characteristics are within the nominal limits specified. Currently available smoke detector systems suffer from the disadvantage of requiring periodic inspection of system hardware and manual adjustment of electrical components to carry out a calibration sequence.
The canopy covering the emitter and sensor is an important hardware component that has two competing functions to carry out. The canopy must act as an optical block for outside light but permit adequate smoke particle intake and flow into the interior of the canopy for interaction with the emitter and sensor. The canopy must also be constructed to prevent the entry of insects and dust, both of which can affect the optical response of the system and its ability to respond to a valid alarm condition. The interior of the canopy should be designed so that secondary reflections of light occurring within the canopy are either directed away from the sensor and out of the canopy or absorbed before they can reach the sensor.